High-frequency vibrations, so-called structure-borne sound, regularly develop in the material of the line itself in line arrangements such as exhaust lines of motor vehicles when the line arrangements in question are connected to a source generating vibrations, such as an engine or the like. The high-frequency vibrations are generated by both residual imbalances of moving parts, such as rotors of exhaust gas turbochargers or compressors, as well as by pressure pulsations in the line arrangement, for example, during the opening of exhaust valves of an engine. The structure-borne sound induced hereby propagates in the exhaust system and leads to an undesired emission of noises in the audible acoustic range in downstream devices, such as catalytic converters or mufflers.
Line elements made of metal for uncoupling high-frequency vibrations, as they are shown, for example, in EP 1 026 375 A2, are known according to the prior art for avoiding the problem of noise emission. The uncoupling is brought about accordingly in a line arrangement with a short, helical or annularly corrugated bellows with high rigidity inserted by a knitted fabric jacket placed in a non-positive or positive-locking manner being arranged on a profiled section of the bellows. However, it was found to be especially disadvantageous in such line elements that the arrangement in space of the knitted fabric jacket leads to an increase in the radial extension of the line arrangement, which collides with the crowded space conditions given in exhaust systems of motor vehicles especially when it is used in such exhaust systems. In addition, the knitted fabric jacket according to EP 1 026 375 A2 touches the bellows based on its arrangement in area in which the bellows has an especially high flexural rigidity based on its corrugated design, so that sufficient uncoupling of high-frequency vibrations cannot be achieved by such an arrangement of the damping knitted fabric jacket.
In German Patent Application DE 10 2004 025 946 B4, the bellows is the only connection between the ends of the line element. Damping pads placed on the bellows end half corrugations, with which the components of the transverse bending vibrations of the structure-borne sound that vibrate at right angles to the bellows surface are already damped markedly more effectively than this is achieved by the knitted fabric jacket placed in a positive-locking manner in the above-described EP 1 026 375 A2, are used as damping elements.
The discovery that the uncoupling of structure-borne sound, which is, in reality, a reduction of the sound intensity passed through, is achieved by diffraction, reflection or by interferences rather than primarily by attenuation by means of damping of the vibrations, as this happened in the above-mentioned documents, is the main topic of the structure-borne sound uncoupling element according to Utility Model DE 20 2007 003 805 U1. Diffractions, reflections and interferences are correspondingly generated above all by the specific introduction of inhomogeneities and discontinuities in the metal bellows by means of a corresponding design of a vibration-affecting bellows layer, which may be provided with holes, slots, impressions or small, applied metal or ceramic plates.
The proposed inhomogeneities and discontinuitities in the metal bellows lead to a considerable extra effort in connection with the manufacture of the bellows. In addition, there must, in addition, be a sealing layer in the bellows, which must be made without vibration-affecting holes and slots. Thus, a bellows layer is still present, through which the structure-borne sound can pass unhindered, because no vibration-affecting measures act in this layer.
A line element, which has a metal foam element in the form of a plated metal foam ring at at least one end of the bellows between the bellows end and the continuing pipeline, is shown in German Utility Model DE 20 2011 005 302 U1. A section, which causes multiple deflections of sound propagation due to its labyrinthine structure, is inserted in this manner in the path of the structure-borne sound. A marked reduction of the sound intensity passed through takes place here due to reflections, diffraction and interferences, even though this dissipation does not meet the increased requirements imposed on structure-borne sound damping.